The invention relates to a semi-submersible floating offshore structure suitable for use in deep waters for supporting, for instance oil and gas drilling and production operations.
In recent years, various types of structures have been developed for use in oil and gas exploration in waters more and more distant from shore, demanding vessel designs that can operate in deep waters, preferably free standing offshore platforms and floating vessels. Some of the known designs use fixed bottom supported structures, with legs embedded into the ocean floor, while others provide for drilling from floating structures, particularly in deeper water.
Currently available floating designs include drill ships, conventional semi-submersible units, as well as tension leg platforms. All of these design approaches have advantages, as well as apparent disadvantages. For instance, drill ships have poor motion characteristics, particularly in quartering or beam sea conditions; conventional semi-submersible vessels are expensive to fabricate and are sensitive to topside changes in weights, wave and wind forces and direction; fixed platforms cannot be moved from one location to another and therefore are considered a poor economic investment for small field production in deep waters. Tension leg platforms are very sensitive to changes in topside weights and area and require permanent type footings on the sea floor, which cannot be easily moved.
All structures are exposed to ocean waves, and it is common knowledge that conventional vessels exhibit substantial amounts of vertical heave, horizontal surge and angular roll and pitch motions. The heave and roll motion is partially alleviated by semi-submersible vessels, which are supported by buoyant floatation columns, tanks and damping action of the submerged pontoons. However, a conventional semi-submersible vessel requires a larger water plane area at the water line in order to maintain positive stability, thus making it more responsive to surface waves and winds. Tension leg platforms reduce heave and roll motions but require strong mooring tendons to resist the vertical and lateral forces acting upon them.
The present invention contemplates reducing or eliminating many of the drawbacks associated with other conventional deep water structures, and providing a mobile, stable structure, having superior motion characteristics and reduced response to wind and wave forces.